System and method of sensing actuation and release voltages of an interferometric modulator

ABSTRACT

A method for sensing the actuation and/or release voltages of a microelectromechanical device include applying a varying voltage to the device and sensing its state and different voltage levels. In one embodiment, the device is part of a system comprising an array of interferometric modulators suitable for a display. The method can be used to compensate for temperature dependent changes in display pixel characteristics.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/604,892, titled “SENSING STATUS OF A MEMS MEMORY DEVICE”, filed Aug.27, 2004 which is hereby incorporated by reference, in its entirety.

BACKGROUND

1. Field

The field of the invention relates to microelectromechanical systems(MEMS).

2. Description of the Related Technology

Microelectromechanical systems (MEMS) include micro mechanical elements,actuators, and electronics. Micromechanical elements may be createdusing deposition, etching, and or other micromachining processes thatetch away parts of substrates and/or deposited material layers or thatadd layers to form electrical and electromechanical devices. One type ofMEMS device is called an interferometric modulator. An interferometricmodulator may comprise a pair of conductive plates, one or both of whichmay be partially transparent and capable of relative motion uponapplication of an appropriate electrical signal. One plate may comprisea stationary layer deposited on a substrate, the other plate maycomprise a metallic membrane suspended over the stationary layer. Suchdevices have a wide range of applications, and it would be beneficial inthe art to utilize and/or modify the characteristics of these types ofdevices so that their features can be exploited in improving existingproducts and creating new products that have not yet been developed.

SUMMARY

The system, method, and devices of the invention each have severalaspects, no single one of which is solely responsible for its desirableattributes. Without limiting the scope of this invention, its moreprominent features will now be discussed briefly. After considering thisdiscussion, and particularly after reading the section entitled“Detailed Description of Certain Embodiments” one will understand howthe features of this invention provide advantages over other displaydevices.

In one embodiment, the invention includes method of determining one orboth of an actuation voltage and a release voltage of amicroelectromechanical device. Such a method may include applying atleast two different electric potentials to at least one electrodecoupled to the device and detecting at least one capacitance dependentresponse of the device at the at least two different electricpotentials. Based at least in part on the response, a state of thedevice at the at least two different electric potentials may bedetermined. From this measurement, one or both of the actuation voltageand release voltages may be determined.

In another embodiment, a display system includes an array ofmicroelectromechanical pixels configured to present display data to auser of the display system. In addition, at least one additionalmicroelectromechanical pixel is provided. Furthermore, a sensor isprovided that is configured to sense one or both of an actuation voltageor a release voltage of the additional microelectromechanical pixel

In another embodiment, the invention includes a method for compensatingfor shifts in one or both of the release and actuation voltage displaypixels. In this embodiment, determining the actuation and or releasestate of at least one pixel is determined, and in response, drivingvoltage levels are modified. The method may be used for compensating fora temperature dependence of a display device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable mirror of a firstinterferometric modulator is in a reflective, or “on,” position at apredetermined distance from a fixed mirror and the movable mirror of asecond interferometric modulator is in a non-reflective, or “off”position.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3A is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 3B is an illustration of sets of row and column voltages that maybe used to drive an interferometric modulator display.

FIGS. 4A and 4B illustrate one exemplary timing diagram for row andcolumn signals that may be used to write a frame of display data to the3×3 interferometric modulator display of FIG. 2.

FIG. 5A is a cross section of the device of FIG. 1.

FIG. 5B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 5C is a cross section of an alternative embodiment of aninterferometric modulator

FIG. 6 is a schematic/block diagram of one embodiment of a state sensingcircuit.

FIG. 7 is graph illustrating a voltage vs. time response to a voltagepulse for an interferometric modulator.

FIG. 8 is a schematic/block diagram of another embodiment of a statesensing circuit

FIG. 9 is graph illustrating a current vs. time response to a voltagepulse for an interferometric modulator.

FIG. 10 is a flow chart of a state sensing process.

FIG. 11 is a timing diagram illustrating row and column voltages forsetting and testing a row of interferometric modulators.

FIG. 12 is a block diagram of a state sensing apparatus for modulatorsembedded in arrays.

FIG. 13 is a flow chart of another embodiment of a state sensingprocess.

FIG. 14 is a block diagram of a display incorporating test pixels.

FIG. 15 is a graph of voltage versus time applied to a pixel which maybe used to determine the actuation and release voltages of aninterferometric modulator.

FIG. 16 is a schematic of a circuit which may be used to determine theactuation and release voltages used with the voltage versus time voltageapplication of FIG. 15.

FIG. 17 is a graph illustrating the timing of the circuit of FIG. 16.

FIG. 18 is a graph of another embodiment of a voltage versus timeapplied to a pixel and current versus time response which may be used todetermine the actuation and release voltages of an interferometricmodulator

FIG. 19 is a schematic of a circuit which may be used to determine theactuation and release voltages with the voltage versus time voltageapplication of FIG. 18.

FIG. 20 is a schematic of another embodiment of a circuit which may beused to determine the actuation and release voltages of aninterferometric modulator.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following detailed description is directed to certain specificembodiments of the invention. However, the invention can be embodied ina multitude of different ways as defined and covered by the claims. Inthis description, reference is made to the drawings wherein like partsare designated with like numerals throughout.

Embodiments of the invention may be implemented in any device that isconfigured to display an image, whether in motion (e.g., video) orstationary (e.g., still image), and whether textual or pictorial. Moreparticularly, it is contemplated that the invention may be implementedin or associated with a variety of electronic devices such as, but notlimited to, mobile telephones, wireless devices, personal dataassistants (PDAs), hand-held or portable computers, electronic books,GPS receivers/navigators, cameras, MP3 players, camcorders, gameconsoles, wrist watches, clocks, calculators, television monitors, flatpanel displays, computer monitors, auto displays (e.g., odometerdisplay, etc.), cockpit controls and/or displays, display of cameraviews (e.g., display of a rear view camera in a vehicle), electronicphotographs, electronic billboards or signs, projectors, architecturalstructures, packaging, and aesthetic structures (e.g., display of imageson a piece of jewelry).

MEMS devices of similar structure to those described herein can also beused in non-display applications such as in electronic switchingdevices.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, the pixels are in either a bright or dark state. In the bright(“on” or “open”) state, the display element reflects a large portion ofincident visible light to a user. When in the dark (“off” or “closed”)state, the display element reflects little incident visible light to theuser. Depending on the embodiment, the light reflectance properties ofthe “on” and “off” states may be reversed. MEMS pixels can be configuredto reflect predominantly at selected colors, allowing for a colordisplay in addition to black and white.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a pair of reflectivelayers positioned at a variable and controllable distance from eachother to form a resonant optical cavity with at least one variabledimension. In one embodiment, one of the reflective layers may be movedbetween two positions. In the first position, referred to herein as thereleased state, the movable layer is positioned at a relatively largedistance from a fixed partially reflective layer. In the secondposition, the movable layer is positioned more closely adjacent to thepartially reflective layer. Incident light that reflects from the twolayers interferes constructively or destructively depending on theposition of the movable reflective layer, producing either an overallreflective or non-reflective state for each pixel.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12 a and 12 b. In the interferometricmodulator 12 a on the left, a movable and highly reflective layer 14 ais illustrated in a released position at a predetermined distance from afixed partially reflective layer 16 a. In the interferometric modulator12 b on the right, the movable highly reflective layer 14 b isillustrated in an actuated position adjacent to the fixed partiallyreflective layer 16 b.

The fixed layers 16 a, 16 b are electrically conductive, partiallytransparent and partially reflective, and may be fabricated, forexample, by depositing one or more layers each of chromium andindium-tin-oxide onto a transparent substrate 20. The layers arepatterned into parallel strips, and may form row electrodes in a displaydevice as described further below. The movable layers 14 a, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes 16 a, 16 b) deposited on top ofposts 18 and an intervening sacrificial material deposited between theposts 18. When the sacrificial material is etched away, the deformablemetal layers are separated from the fixed metal layers by a defined airgap 19. A highly conductive and reflective material such as aluminum maybe used for the deformable layers, and these strips may form columnelectrodes in a display device.

With no applied voltage, the cavity 19 remains between the layers 14 a,16 a and the deformable layer is in a mechanically relaxed state asillustrated by the pixel 12 a in FIG. 1. However, when a potentialdifference is applied to a selected row and column, the capacitor formedat the intersection of the row and column electrodes at thecorresponding pixel becomes charged, and electrostatic forces pull theelectrodes together. If the voltage is high enough, the movable layer isdeformed and is forced against the fixed layer (a dielectric materialwhich is not illustrated in this Figure may be deposited on the fixedlayer to prevent shorting and control the separation distance) asillustrated by the pixel 12 b on the right in FIG. 1. The behavior isthe same regardless of the polarity of the applied potential difference.In this way, row/column actuation that can control the reflective vs.non-reflective pixel states is analogous in many ways to that used inconventional LCD and other display technologies.

FIGS. 2 through 4 illustrate one exemplary process and system for usingan array of interferometric modulators in a display application. FIG. 2is a system block diagram illustrating one embodiment of an electronicdevice that may incorporate aspects of the invention. In the exemplaryembodiment, the electronic device includes a processor 21 which may beany general purpose single- or multi-chip microprocessor such as an ARM,Pentium®, Pentium II®, Pentium III®, Pentium IV®, Pentium® Pro, an 8051,a MIPS®, a Power PC®, an ALPHA®, or any other suitable processor. Inaddition, the processor 21 may comprise any special purposemicroprocessor such as a digital signal processor, microcontroller, or aprogrammable gate array. As is conventional in the art, the processor 21may be configured to execute one or more software modules. In additionto executing an operating system (not shown), the processor may beconfigured to execute one or more software applications, including a webbrowser, a telephone application, an email program, or any othersoftware application. It will be appreciated that all of thefunctionality described herein may be implemented in whole or part inhardware, software, or a combination thereof.

In one embodiment, the processor 21 is also configured to communicatewith an array controller 22. In one embodiment, the array controller 22includes a row driver circuit 24 and a column driver circuit 26 thatprovide signals to the array 30. The cross section of the arrayillustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. The arraycontrol 22 may also include a boost circuit 32 for converting controlsignals to a voltage or voltages sufficient for driving the array 30. Inone embodiment, the display control 22 also includes a frame buffer 34.The frame buffer typically includes sufficient memory to store thecurrent displayed frame for refresh purposes.

A plurality of tri-state buffers 36 are advantageously provided on eachof the columns and each of the rows of array 30. The tri-state buffers36 are connected to hold-mode signals which open the connection to therespective row or column of the array when they are asserted. When thehold-mode select lines are asserted, lines from the driver to the arrayare opened, substantially eliminating any leakage path for the chargestored on each pixel capacitance. The pixels are thus held in thepreviously charged or discharged state without any driver input, untilthe charge slowly dissipates, either through leakage across the pixel orthrough a non-infinite tri-state open resistance. It will be appreciatedthat any controllable series switch such as a series FET could be usedto implement this display/driver decoupling.

For MEMS interferometric modulators, the row/column actuation protocolmay take advantage of a hysteresis property of these devices illustratedin FIG. 3A. It may require, for example, a 10 volt potential differenceto cause a movable layer to deform from the released state to theactuated state. However, when the voltage is reduced from that value,the movable layer maintains its state as the voltage drops back below 10volts. In the exemplary embodiment of FIG. 3A, the movable layer doesnot release completely until the voltage drops below 2 volts. There isthus a range of voltage, about 3 to 7 V in the example illustrated inFIG. 3A, where there exists a window of applied voltage within which thedevice is stable in either the released or actuated state. This isreferred to herein as the “hysteresis window” or “stability window.” Fora display array having the hysteresis characteristics of FIG. 3A, therow/column actuation protocol can be designed such that during rowstrobing, pixels in the strobed row that are to be actuated are exposedto a voltage difference of about 10 volts, and pixels that are to bereleased are exposed to a voltage difference of close to zero volts.After the strobe, the pixels are exposed to a steady state voltagedifference of about 5 volts such that they remain in whatever state therow strobe put them. After being written, each pixel sees a potentialdifference within the “stability window” of 3-7 volts in this example.This feature makes the pixel design illustrated in FIG. 1 stable underthe same applied voltage conditions in either an actuated or releasedpre-existing state. Since each pixel of the interferometric modulator,whether in the actuated or released state, is essentially a capacitorformed by the fixed and moving reflective layers, this stable state canbe held at a voltage within the hysteresis window with almost no powerdissipation. Essentially no current flows into the pixel if the appliedpotential is fixed.

In typical applications, a display frame may be created by asserting theset of column electrodes in accordance with the desired set of actuatedpixels in the first row. A row pulse is then applied to the row 1electrode, actuating the pixels corresponding to the asserted columnlines. The asserted set of column electrodes is then changed tocorrespond to the desired set of actuated pixels in the second row. Apulse is then applied to the row 2 electrode, actuating the appropriatepixels in row 2 in accordance with the asserted column electrodes. Therow 1 pixels are unaffected by the row 2 pulse, and remain in the statethey were set to during the row 1 pulse. This may be repeated for theentire series of rows in a sequential fashion to produce the frame.Generally, the frames are refreshed and/or updated with new display databy continually repeating this process at some desired number of framesper second. A wide variety of protocols for driving row and columnelectrodes of pixel arrays to produce display frames are also well knownand may be used in conjunction with the present invention.

FIGS. 3B and 4 illustrate one possible actuation protocol for creating adisplay frame on the 3×3 array of FIG. 2. FIG. 3B illustrates a possibleset of column and row voltage levels that may be used for pixelsexhibiting the hysteresis curves of FIG. 3A. In the FIG. 3B embodiment,actuating a pixel involves setting the appropriate column to −V_(bias),and the appropriate row to +ΔV, which may correspond to −5 volts and +5volts respectively Releasing the pixel is accomplished by setting theappropriate column to +V_(bias), and the appropriate row to the same+ΔV, producing a zero volt potential difference across the pixel. Inthose rows where the row voltage is held at zero volts, the pixels arestable in whatever state they were originally in, regardless of whetherthe column is at +V_(bias), or −V_(bias).

FIG. 4B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 4A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 4A, thepixels can be in any state, and in this example, all the rows are at 0volts, and all the columns are at +5 volts. With these applied voltages,all pixels are stable in their existing actuated or released states.

In the FIG. 4A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and releases the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumns 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and release pixels (2,1) and (2,3). Again,no other pixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 4A. It will be appreciated that the same procedurecan be employed for arrays of dozens or hundreds of rows and columns. Itwill also be appreciated that the timing, sequence, and levels ofvoltages used to perform row and column actuation can be varied widelywithin the general principles outlined above, and the above example isexemplary only, and any actuation voltage method can be used with thepresent invention.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 5A-5C illustrate three different embodiments of themoving mirror structure. FIG. 5A is a cross section of the embodiment ofFIG. 1, where a strip of metal material 14 is deposited on orthogonallyextending supports 18. In FIG. 5B, the moveable reflective material 14is attached to supports at the corners only, on tethers 38. In FIG. 5C,the moveable reflective material 14 is suspended from a deformable layer40. This embodiment has benefits because the structural design andmaterials used for the reflective material 14 can be optimized withrespect to the optical properties, and the structural design andmaterials used for the deformable layer 40 can be optimized with respectto desired mechanical properties. The production of various types ofinterferometric devices is described in a variety of publisheddocuments, including, for example, U.S. Published Application2004/0051929. A wide variety of well known techniques may be used toproduce the above described structures involving a series of materialdeposition, patterning, and etching steps.

After a pixel is written, it can be advantageous to sense its state. Forthe bi-stable display of FIG. 1, the state of a pixel can be determinedby taking advantage of the fact that the capacitance across the pixelmirrors is much larger, often about ten times larger, when the pixelsare in the actuated state than when they are in the released state. Thispixel capacitance value can be sensed in a variety of ways by sensingcapacitance dependent electrical properties of the pixel, some of whichare described in more detail below.

The principles of pixel state sensing will be described first withreference to a single pixel in isolation as illustrated in FIGS. 6-10.Referring now to FIG. 6, after pixel writing, whether the whole frame iscomplete or prior to that time, all the column tri-state buffers can beplaced in the open (decoupled) configuration except one columncontaining the pixel to be tested. The row driver then applies a lowamplitude pulse to the row electrode containing the pixel to be tested,which charges up in response to the increased voltage. As shown in FIG.7, the voltage across the pixel will increase in response to thisapplied voltage in accordance with the RC time constant (τ) of thecircuit. For a single pixel in isolation, the capacitance is thecapacitance of the pixel 54, and the resistance of the circuit mayinclude the row driver output impedance and/or any filter resistor 56that might be placed in series with the row electrode. The voltage atthe test point 58 when the pixel 54 is in a low capacitance state (e.g.in the released state) will increase faster as illustrated by curve 60than when the pixel 54 is in a high capacitance state (e.g. in theactuated state) as illustrated by curve 62. If the voltage across thepixel is determined at a certain time during this charging period, atτ/3 for example, the state of the pixel can be determined. This voltagecan be detected and measured by a voltage sensing circuit 64. If a pulsehaving a duration of τ/3 is applied to the pixel, the voltage across thepixel will increase and decrease as shown in the trace 66 (also shown inFIG. 7). If this signal is applied to the input of a comparator 68 withV_(thresh) applied to the negative input, a pulse will be output fromthe comparator only if the voltage across the pixel exceeded V_(thresh)at some time during the pulse, where V_(thresh) is defined as shown inFIG. 7. The output of the comparator 68 can be latched to produce anindication of whether that pixel is actuated (latch low) or released(latch high).

FIGS. 8 and 9 illustrate an alternative method of detecting pixel state.In FIG. 8, a current sensing circuit 70 is used rather than a voltagesensing circuit. A voltage pulse is applied as above, which causes acurrent pulse as the pixel capacitance charges. As illustrated in FIG.9, this current pulse decays slower (curve 75) for a larger capacitanceof pixel 54 than for a smaller capacitance (curve 77). The current pulsecan be converted to a voltage pulse by measuring the voltage across aseries resistance 72 in the column line (amplifiers configured ascurrent to voltage converters could also be used). The voltage acrossthe resistor can be sensed by an amplifier configured as an integrator74 illustrated in FIG. 8. The output of the integrator can be routed toa similar comparator 76 and latch as in FIG. 6. The comparator 76 willonly produce an output pulse if the current pulse through the circuit issufficient (given the value of the resistor 72 and the timeconstant/amplification of the integrator 74) to produce a voltage at thecomparator input greater than a threshold voltage V_(thresh2) shown inFIG. 8. FIG. 8 shows a switch 78 used to switch resistance 72 into thecolumn line, but it will be appreciated that this would not be necessaryif a suitable filter resistor, for example, was already present.

Current sensing requires a slightly more complicated circuit thanvoltage sensing, but one advantage would be that all the pixels in a rowcould be probed by a single pulse since the charging current could beseparately measured for each pixel along a row simultaneously withseparate current sensors. In these embodiments, there may be a sensordedicated to each column, or a set of current sensors could besequentially switched between different groups of columns such that aportion, but not all of the column currents are sensed concurrently.This last embodiment would be slower than an embodiment with a sensorfor every row, but faster than one at a time sensing.

In accordance with the principles above, FIG. 10 is a flowchartillustrating an exemplary process for determining an open or closedstate of an interferometric modulator. A test pulse is applied to thepixel at step 80. At step 82, a capacitance dependent response to thepulse is measured. At step 84, the response is compared to a thresholdto determine the state of the pixel.

Pixel state sensing can be advantageous for a variety of reasons. Forexample, on the next frame update or refresh, only those pixels that aredifferent from the next desired frame need be updated. For a staticdisplay, the pixel states may be monitored to detect which pixels haverelaxed from an actuated state to a released state due to chargeleakage. Selective updating could be performed in a variety of ways. Forexample, once one or more pixels change from the desired state, thedriver circuitry could be turned back on, the tri-state buffers closed,and row strobing could be limited to only those rows which includepixels in an undesired state. Other rows could be skipped. This reducesthe total energy required to update the display. Pixel state sensingcould also be advantageous during the frame writing process, because asrows of pixels are written, they could be checked to determine if theywere written correctly or not. If not, the row could be written againuntil correct. Pixel state sensing can also advantageously minimize thepeak memory requirements for the frame buffer.

An implementation of this last process is illustrated in FIG. 11. Afterwriting row 1 during the row 1 line time 90, a row 1 test time 92 isentered. In the first portion of this time period, only row 1 and column1 are connected to the drive circuitry, and a test pulse 94 of about 1volt or less is applied to row 1. As described above, the capacitancedependent response of pixel (1,1) is monitored to be sure it is in theactuated state as shown in FIG. 5A. This is repeated for pixels (1,2)and (1,3) during subsequent portions of the row 1 test time. The systemthen enters the row 2 line time, or alternatively, repeats the row 1line time if it is determined that one or more pixels in row 1 have notbeen correctly written. For purposes of illustration, the test voltageamplitude is shown larger than generally desired and the test timeperiod is shown much longer than would normally be necessary, as thepulse time periods for testing can be very short compared to the pulseperiods used to actuate the pixels during the write process. When thepixel 54 being tested is part of a large array of tightly packed pixels,the testing process may be somewhat more complex. This is because thetest pulse is applied to an entire row of pixels. Thus, the timeconstant of the charging process is dependent on the capacitance betweenthe entire row electrode and the return column electrode, and this canbe affected by the relative states of all the pixels in the row, notjust on the state of the pixel being tested 54, shown again in FIG. 12.The dominant factor in the capacitance will be the state of the pixelbeing tested, but since there may be hundreds of pixels in the row, thecombined effect of the remainder can be significant. There can also becapacitive coupling between pixels in different rows that share the samecolumn electrode. The practical effect of this is that it may beadvantageous to vary the pulse time period τ/3, the V_(thresh) value, orboth, when testing pixels in a given row, depending on the states of theother pixels in the row.

This determination can be done in several ways. One embodimentillustrated in FIG. 12 can include in each row, at the end of the rowoutside the viewed area of the display, a test pixel 98. This pixel canbe switched between states, and the rise times for the test pulse can bedetermined for both the actuated and released states. In this way, thetime period having the maximum voltage difference between states, andthe voltage values between which V_(thresh) should be located could bedetermined based on the test pixel response. These values could then beused to test the state of the other pixels in the row.

Alternatively, a filter resistor could be placed at the end of the rowinstead of a test pixel. A collective capacitance measurement for thewhole row electrode could then be made. The drive control circuitrycould use this information to compute or look up an appropriate valuefor τ/3, V_(thresh), or both, to test the pixels in that row.

A general state sensing process using these principles for pixelsembedded in arrays of rows and columns is illustrated in FIG. 13. Atstep 102, row measurement signals are applied to a row containing apixel to be sensed. These signals could involve testing a test pixel oran overall row capacitance measurement as described above. At step 104,appropriate test parameters such as period τ/3 and/or V_(thresh) aredetermined for later pixel testing in the row. As in FIG. 10, a testpulse is then applied to the row at step 106. At step 108, a capacitancedependent response to the pulse is measured. At step 110, the responseis compared to a threshold to determine the state of a selected pixel inthe row.

Pulse amplitudes and durations for the pixel state sensing process maybe selected based on a variety of desired factors. The pulse may beshaped to control the total charge injected into the row. For isolatedpixels, the pulse current and time profile can be defined such that apre-selected charge is injected into the pixel regardless of itscapacitance value. In this case, the resulting voltage across the pixelwill be inversely proportional to the pixel capacitance. It may bepossible to use this method for pixels in an array as well, but itsusefulness may be limited since the charge injected into the row may bedistributed throughout the hundreds of row pixels in a way that iscomplicated and difficult to predict. Pulse durations may be selectedbased on the circuit τ value, with short pulses preferred for timesavings. It is of course desirable that the potential applied to thepixel during this process remains at all times within the hysteresiswindow so that the state sensing process does not itself change thestate of the pixel being sensed. Thus, the driver will advantageouslysupply the appropriate bias voltage when not applying a charging pulseand when not decoupled by the tri-state amplifiers, and will generatepulses deviating from this bias voltage that are small enough (e.g. nomore than 1 or 2 volts typically) such that the applied pixel voltagesare never outside of the hysteresis window.

Another advantageous application of pixel state sensing is fordetermining the actuation and release voltages of a pixel. This can beuseful because these voltages are temperature dependent, and may shiftover time as well. Higher temperatures tend to shift the stabilitywindow of FIG. 3A closer to zero for metal mirrors on glass substrates.Depending on the relative coefficients of thermal expansion of thematerial layers, shifts of either direction as a function of temperaturecan occur. If pixel actuation and release voltages can be determinedelectrically, the drive voltages used to write image data to an array ofpixels can be modified to match the current pixel behavior. A displayincorporating this feature is illustrated in FIG. 14. In thisembodiment, extra test pixels 112 are placed around the actual viewingarea of the display 114. These test pixels may be fabricated during thesame process that produces the display so that the physicalcharacteristics are similar if not essentially identical with thephysical characteristics of the pixels in the viewed display area 114.One or more sense circuits 118 that apply variable bias voltages andtest voltages are coupled to the test pixels. It will be appreciatedthat some or all of the sense circuitry could be shared among multipletest pixels.

With separate test pixels, a variety of sensing protocols can beimplemented to determine the actuation and/or the release voltages of acapacitive MEMS pixel. For example, this determination can be performedby applying a series of voltages across a pixel, and sensing the stateat each applied voltage. This is illustrated in FIG. 15. The voltage canbe stepped up from zero to a voltage that is above the expectedactuation voltage under all conditions. At each voltage level, a pixelstate test as described above may be performed to determine the pixelstate. At some voltage level, the pixel will actuate, and this will bedetected during the test. Pixel voltage can then be stepped down andtested at each level back down to zero. At some voltage level, the pixelwill release, and this will again be detected by the test results.

In FIG. 15, the voltage step is one volt for each step, but it will beappreciated that any step size may be used. During each step, after thepixel has charged from the previous voltage transition, a test pulse 120is applied as described above. The appropriate voltages or currents aremonitored as desired, and the pixel state is determined at each voltagelevel. Ranges for the actuation and release voltages can be determinedby determining which step caused a state change. Advantageously, theamplitude and duration of the test pulses are less than the step sizeand duration.

FIGS. 16 and 17 illustrate a circuit and its operation that canimplement the method of FIG. 15. In this embodiment, a test pulse isadded to a series of stepped up and stepped down voltages and the signalsum is applied to one side of a pixel. The other side is grounded withan inverting current to voltage converter 124. A switch 126 connects theoutput of the current to voltage converter to the input of a comparator128. As shown in FIG. 17, the CLK1 signal attached to the test pulsegenerator produces the test pulse duration. As illustrated in FIG. 9 anddescribed above, the test pulse produces a current pulse through thetest pixel that decays much slower for an actuated pixel than for areleased pixel. The CLK2 signal controls the connection between theoutput of the current to voltage converter 124 and the input to thecomparator 128. The input to the comparator is pulled low by resistor130 when the switch 126 is open. The CLK2 signal is timed to have arising edge delayed from the rising edge of CLK1 and have a shortduration to sample the voltage output from the current to voltageconverter 124 at a selected point in time during the charging process.This voltage will be higher for an actuated pixel than for a releasedpixel. If the voltage is more negative than −Vthresh3 (negative becauseof the inverting amplifier 124) during the CLK2 sample period, thisindicates an actuated pixel, and the output of the comparator 128 willbe high during the sample period. This is repeated sequentially for eachtest pulse, and the series of comparator outputs are shifted into ashift register 136 at times determined by signal CLK3 which is withinthe assertion time of CLK2. The outputs of the shift register 136 thenform a record of the actuated vs. released state of the pixel at eachlevel on the way up and back down.

FIGS. 18 and 19 illustrate another circuit implementation that can beused to determine actuation and release voltages of a bistable pixel. Ifthe voltage on the pixel is increased at a rate that is slow compared tothe pixel RC charging time constant and the time it takes for a pixel toswitch between states, the current will be very low while the voltage isramped up. This will be true until the pixel changes to the actuatedstate and the capacitance quickly increases. This will cause a currentpulse to flow during the transition to a high capacitance state. On theramp back downward, a second current pulse in the opposite direction(quickly reducing the charge on the pixel capacitance) will occur.

These current pulses can be detected by the circuit of FIG. 19. In thisembodiment, the output of the current to voltage converter 124 iscoupled to a pair of comparators 140 and 142. Both comparator outputswill both be low when the charging current is small. During the firstcurrent pulse, the output of comparator 140 will go high. During thesecond current pulse, the output of comparator 142 will go high. Thetime at which these pulses occur can be determined by having each outputpulse from the comparators stop a respective counter 144, 146 that isstarted at the same time the ramp is started. The counter values can beassociated with the actuation and release voltages because the voltageas a function of time of the applied voltage ramp is known.

Another possible test circuit is illustrated in FIG. 20. In thisembodiment, an AC signal is placed on top of a DC bias voltage and isapplied to the pixel at node 150. More AC current will flow through thepixel when the pixel is actuated than when it is released. This ACcurrent can be detected by including both a DC coupled path to groundand an AC coupled path to ground on the other plate of the pixel. The DCvoltage across capacitor 154 will increase with increasing AC currentthrough the pixel and through capacitor 156. This voltage is routed to acomparator 158, which goes high if this value is above Vthresh6, whichis determined based on the component values. In this embodiment, the DCbias voltage can be varied in any manner, and the output 160 of thecomparator 158 will be high when the pixel is actuated, and low when thepixel is released.

While the above detailed description has shown, described, and pointedout novel features of the invention as applied to various embodiments,it will be understood that various omissions, substitutions, and changesin the form and details of the device or process illustrated may be madeby those skilled in the art without departing from the spirit of theinvention. As one example, it will be appreciated that the test voltagedriver circuitry could be separate from the array driver circuitry usedto create the display. As with current sensors, separate voltage sensorscould be dedicated to separate row electrodes. The scope of theinvention is indicated by the appended claims rather than by theforegoing description. All changes which come within the meaning andrange of equivalency of the claims are to be embraced within theirscope.

1. A method of determining one or both of an actuation voltage and arelease voltage of a microelectromechanical device, the methodcomprising: applying at least two different electric potentials to atleast one electrode coupled to said device; detecting at least onecapacitance dependent response of said device at said at least twodifferent electric potentials; determining, based at least in part onsaid response, a state of said device at said at least two differentelectric potentials; and determining one or both of said actuationvoltage and release voltage based at least in part on said determining astate of said device.
 2. The method of claim 1, wherein said determiningcomprises determining whether said response is greater than a threshold.3. The method of claim 2, wherein said detecting comprises sensing avoltage generated on said electrode.
 4. The method of claim 2, whereinsaid detecting comprises sensing a current flow generated through saidpixel.
 5. A display system comprising: an array ofmicroelectromechanical pixels configured to present display data to auser of said display system; at least one additionalmicroelectromechanical pixel that is not one of saidmicroelectromechanical pixels configured to present display data; and asensor, configured to sense an actuated state or a released state orboth, of said at least one additional microelectromechanical pixel. 6.The display system of claim 5, wherein said sensor senses a capacitancedependent response of said additional microelectromechanical pixels. 7.The display system of claim 6, wherein said sensors comprise currentsensors.
 8. The system of claim 5, wherein at least some of said pixelsconfigured to present display data comprise interferometric modulators.9. The system of claim 5, wherein said sensor comprises a comparator.10. A method for compensating for shifts in one or both of the releaseand actuation voltage display pixels, the method comprising: determiningthe actuation and or release state of at least one pixel; and modifyingdriving voltage levels in response to said determining.
 11. The methodof claim 10, wherein said determining comprises determining theactuation and or release state of a test pixel.
 12. A method ofcompensating for a temperature dependence of a display device, saidmethod comprising determining one or both of an actuation and releasevoltage for at least one pixel associated with said display device andmodifying device operational parameters based at least in part on saiddetermining.
 13. The method of claim 12, wherein said determiningcomprises determining a capacitance dependent electrical response ofsaid at least one pixel.
 14. The method of claim 12, wherein said atleast one pixel comprises a test pixel that is not part of a viewabledisplay.
 15. A display device comprising: an array ofmicroelectromechanical pixels; at least one test microelectromechanicalpixel; and means for determining one or both of an actuation voltage andrelease voltage of said test microelectromechanical pixel.
 16. Thedisplay system of claim 5, wherein said at least one additionalmicromechanical pixel is separate from said array of micromechanicalpixels.
 17. The display system of claim 5, wherein said at least oneadditional micromechanical pixel comprises two or more micromechanicalpixels placed around the actual viewing are of the display.
 18. Thedisplay system of claim 5, wherein said at least one additionalmicromechanical pixel is fabricated during the same process thatproduces said array such that the physical characteristics are similarif not essentially identical with the physical characteristics of thepixels in the viewable array.
 19. The display system of claim 17,wherein some or all of the sense circuitry is shared among said two ofmore micromechanical pixels.
 20. The display system of claim 17, wheresaid at least one additional micromechanical pixel comprises fourmicromechanical pixels each being disposed near a corner of said array.21. A display system comprising: an array of microelectromechanicalpixels configured to present display data to a user of said displaysystem; and a test circuit configured to apply at least two differentelectric potentials to at least one electrode coupled to amicroelectromechanical pixel in said array, detect at least onecapacitance dependent response of said device at said at least twodifferent electric potentials, determine based at least in part on saidresponse, a state of said device at said at least two different electricpotentials, and determine one or both of said actuation voltage andrelease voltage based at least in part on said determining a state ofsaid device.